The present invention relates to a charged particle beam exposure apparatus capable of performing exposure while changing the dimensions of the cross section of a charged particle beam in accordance with an exposure pattern.
Various types of electron beam exposure apparatus are used for forming fine exposure patterns on a semiconductor wafer or mask. The cross-sectional area of an electron beam used in such an apparatus is extremely small and is suitable for forming a fine exposure pattern. However, at the same time, this means that the exposure area per time unit is small, resulting in a very low efficiency in the manufacture of semiconductor devices.
In view of this problem, an exposure apparatus has been proposed wherein two aperture masks respectively having a rectangular aperture are sequentially arranged in the path of the electron beam. The electron beam, after passing the first aperture mask, is deflected in x and y directions by a beam deflector interposed between the two aperture masks. Thus, portions of the electron beam are cut by the edges of the second aperture mask defining its aperture. In this manner, this apparatus allows varying of the cross-sectional area or beam spot dimensions of the electron beam bombarded against an object to be exposed. In this case, of the four sides of a rectangular beam pattern radiated onto a semiconductor wafer, one pair of adjacent sides are defined by the edges of the second aperture and do not therefore change in position after beam deflection. On the other hand, the remaining two adjacent sides change in length upon beam deflection, thereby changing the dimensions of the cross section of the beam in the x and y directions. Accordingly, when a beam dimension is to be changed, it is convenient to set an intersection of two sides which do not move upon beam deflection as a reference coordinate point, and to set the beam dimensions in the x and y directions with reference to this reference coordinate point. Then, a pattern of a wide area may be exposed at high speed with a beam of increased dimensions. An apparatus of this type has a wide range of applications since it allows easy reduction in the beam dimensions as needed.
However, according to experiments conducted by the present inventors, an apparatus of the type described above which is capable of varying the beam dimensions is subject to the following problems.
Problem (1)
When the dimensions of an electron beam cross section are changed in the x and y directions, the position of the electron beam, that is, the reference coordinate point which is an intersection of the two adjacent sides of the exposure pattern corresponding to the edges of the aperture of the second aperture mask, changes.
Problem (2)
Further, the rate of the positional change of the reference coordinate point with respect to the dimensional change of electron beam cross section depends on the amount of variation .DELTA.I of a beam current in response to a dimensional change .DELTA.x. When the amount of shift of the reference coordinate point in the x direction is .DELTA.x, and when the beam dimension in the X direction changes by .DELTA.X, the rate becomes .DELTA.x/.DELTA.X, for example. This will be shown in FIG. 4A.
Problem (3)
Furthermore, as the focal point of an electron lens for focusing the electron beam is shifted from the surface of an object to be exposed in the forward or backward direction, the beam position is also shifted. This will be seen in FIG. 4C later. Such shifts in beam position result in degradation in dimension and position precision of the exposure pattern.
Such shifts in beam position are considered to be attributable to a combination of an astigmatism of an objective lens, Coulomb repulsion between electrons in an electron beam passing through each lens, focal point preset error of each lens and so on. Although elimination of these factors theoretically results in the elimination of shifts in beam position, factors determining the astigmatism may be present at various positions on the beam path due to the complex structure of an electron beam exposure apparatus capable of changing beam dimensions. An apparatus free from factors causing astigmatism is very costly.
Defocusing caused by Coulomb repulsion can be corrected by a correction lens. However, the lens must be driven at a high speed in accordance with the dimensions of the beam spot. This also results in a complex manufacturing process and an expensive apparatus.